Running Hadoop MapReduce on Alluxio

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This guide describes how to get Alluxio running with Apache Hadoop MapReduce, so that you can easily run your MapReduce programs with files stored on Alluxio.

Prerequisites

  • Alluxio has been set up and is running.
  • Make sure that the Alluxio client jar is available. This Alluxio client jar file can be found at /<PATH_TO_ALLUXIO>/client/alluxio-2.0.0-client.jar in the tarball downloaded from Alluxio download page. Alternatively, advanced users can compile this client jar from the source code by following the instructions.
  • In order to run map-reduce examples, we also recommend you download the map-reduce examples jar based on your Hadoop version, or if you are using Hadoop 1, this examples jar.

Basic Setup

Configuring Hadoop Core-site Properties

Note that, this step is only required for Hadoop 1.x and can be skipped by users of Hadoop 2.x or later. Add the following property to the core-site.xml file of your Hadoop installation:

<property>
  <name>fs.alluxio.impl</name>
  <value>alluxio.hadoop.FileSystem</value>
  <description>The Alluxio FileSystem</description>
</property>

This will allow your MapReduce jobs to recognize URIs with Alluxio scheme alluxio:// in their input and output files.

Distributing the Alluxio Client Jar

In order for the MapReduce applications to read and write files in Alluxio, the Alluxio client jar must be distributed on the classpath of the application across different nodes.

You can use the -libjars command line option when using hadoop jar ..., specifying /<PATH_TO_ALLUXIO>/client/alluxio-2.0.0-client.jar as the argument of -libjars. Hadoop will place the jar in the Hadoop DistributedCache, making it available to all the nodes. For example, the following command adds the Alluxio client jar to the -libjars option:

./bin/hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-2.7.3.jar wordcount -libjars /<PATH_TO_ALLUXIO>/client/alluxio-2.0.0-client.jar <INPUT FILES> <OUTPUT DIRECTORY>

Sometimes, you also need to set the HADOOP_CLASSPATH environment variable to make Alluxio client jar available to the client JVM which is created when you run the hadoop jar command:

 export HADOOP_CLASSPATH=/<PATH_TO_ALLUXIO>/client/alluxio-2.0.0-client.jar:${HADOOP_CLASSPATH}

Alternative ways are described in the Advanced Setup

Example

For simplicity, we will assume a pseudo-distributed Hadoop cluster, started by running:

cd $HADOOP_HOME
./bin/stop-all.sh
./bin/start-all.sh

Depending on the Hadoop version, you may need to replace ./bin with ./sbin.

Start Alluxio locally:

./bin/alluxio-start.sh local SudoMount

You can add a sample file to Alluxio to run wordcount on. From your Alluxio directory:

./bin/alluxio fs mkdir /wordcount
./bin/alluxio fs copyFromLocal LICENSE /wordcount/input.txt

This command will copy the LICENSE file into the Alluxio namespace with the path /wordcount/input.txt.

Now we can run a MapReduce job (using Hadoop 2.7.3 as example) for wordcount.

./bin/hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-2.7.3.jar wordcount -libjars /<PATH_TO_ALLUXIO>/client/alluxio-2.0.0-client.jar alluxio://localhost:19998/wordcount/input.txt alluxio://localhost:19998/wordcount/output

After this job completes, the result of the wordcount will be in the /wordcount/output directory in Alluxio. You can see the resulting files by running:

./bin/alluxio fs ls /wordcount/output
./bin/alluxio fs cat /wordcount/output/part-r-00000

Tips:The previous wordcount example is also applicable to Alluxio in HA mode. Please follow the instructions in HDFS API to connect to Alluxio with high availability.

Advanced Setup

Distributing the Alluxio Client Jar

This guide on how to include 3rd party libraries from Cloudera describes several ways to distribute the jars. From that guide, the recommended way to distributed the Alluxio client jar is to use the distributed cache, via the -libjars command line option. Another way to distribute the client jar is to manually distribute it to all the Hadoop nodes.

You could place the client jar /<PATH_TO_ALLUXIO>/client/alluxio-2.0.0-client.jar in the $HADOOP_HOME/lib (may be $HADOOP_HOME/share/hadoop/common/lib for different versions of Hadoop) directory of every MapReduce node, and then restart Hadoop. Alternatively, add this jar to mapreduce.application.classpath system property for your Hadoop deployment to ensure this jar is on the classpath.

Note that the jars must be installed again for each update to a new release. On the other hand, when the jar is already on every node, then the -libjars command line option is not needed.

Customize Alluxio User Properties for All MapReduce Jobs

Alluxio configuration parameters can be added to the Hadoop core-site.xml file to affect all MapReduce jobs. Let us use the setup of Hadoop to interact with the Alluxio service in HA Mode as an example.

When Alluxio is running in HA mode, add the HA mode client configuration in core-site.xml file of your Hadoop installation. For example, if the Alluxio is using internal leader election, add the alluxio.master.rpc.addresses property:

<configuration>
  <property>
    <name>alluxio.master.rpc.addresses</name>
    <value>master_hostname_1:19998,master_hostname_2:19998,master_hostname_3:19998</value>
  </property>
</configuration>

See HA mode client configuration parameters for more details.

Customize Alluxio User Properties for Individual MapReduce Jobs

Hadoop MapReduce users can add "-Dproperty=value" after the hadoop jar or yarn jar command and the properties will be propagated to all the tasks of this job. For example, the following MapReduce job of wordcount sets write type to CACHE_THROUGH when writing to Alluxio:

./bin/hadoop jar libexec/share/hadoop/mapreduce/hadoop-mapreduce-examples-2.7.3.jar wordcount \
-Dalluxio.user.file.writetype.default=CACHE_THROUGH \
-libjars /<PATH_TO_ALLUXIO>/client/alluxio-2.0.0-client.jar \
<INPUT FILES> <OUTPUT DIRECTORY>

Troubleshooting

Logging Configuration

Logs with Hadoop can be modified in many different ways. If you wish to directly modify the log4j.properties file for Hadoop, then you can add or modify appenders within ${HADOOP_HOME}/conf/log4j.properties on each of the nodes in your cluster.

You may also modify the configuration values in mapred-site.xml in your installation. If you simply wish to modify log levels then your can change mapreduce.map.log.level or mapreduce.reduce.log.level.

If you arn using YARN then you may also wish to modify some of the yarn.log.* properties which can be found in yarn-site.xml

Check MapReduce with Alluxio integration (Supports Hadoop 2.X)

Before running MapReduce on Alluxio, you might want to make sure that your configuration has been setup correctly for integrating with Alluxio. The MapReduce integration checker can help you achieve this.

When you have a running Hadoop cluster (or standalone), you can run the following command in the Alluxio project directory:

integration/checker/bin/alluxio-checker.sh mapreduce

You can use -h to display helpful information about the command. This command will report potential problems that might prevent you from running MapReduce on Alluxio.

Q: Why do I see exceptions like “No FileSystem for scheme: alluxio”?

A: This error message is seen when your MapReduce application tries to access Alluxio as an HDFS-compatible file system, but the alluxio:// scheme is not recognized by the application. Please make sure your HDFS configuration file core-site.xml has the following property:

<configuration>
  <property>
    <name>fs.alluxio.impl</name>
    <value>alluxio.hadoop.FileSystem</value>
  </property>
</configuration>

Q: Why do I see exceptions like “java.lang.RuntimeException: java.lang.ClassNotFoundException: Class alluxio.hadoop.FileSystem not found”?

A: This error message is seen when your MapReduce application tries to access Alluxio as an HDFS-compatible file system, the alluxio:// scheme has been configured correctly but the Alluxio client jar is not found on the classpath of your application.

You can append the client jar to $HADOOP_CLASSPATH:

export HADOOP_CLASSPATH=/<PATH_TO_ALLUXIO>/client/alluxio-2.0.0-client.jar:${HADOOP_CLASSPATH}

If the corresponding classpath has been set but exceptions still exist, users can check whether the path is valid by:

ls /<PATH_TO_ALLUXIO>/client/alluxio-2.0.0-client.jar